Of course government can change law - but some changes may be foolish - and as Liz Truss has recently seen, messing with the markets too dramatically can have some interesting consequences!
However, there is a fundamental basis that you don't charge tax on expenses - i.e. you don't tax the purchaser directly, simply because you need to collect that tax and some purchasers are members of the public with no mechanism for being taxed in that way. e.g. As a business I buy insurance, I can buy from anywhere in the world, so will buy from Ireland as it is cheaper... the government thinks, I want a bit of that and will tax the transaction:
- taxing the seller - there is a mechanism to do that and collect tax, but the Irish seller is out of the government's reach...
- taxing the buyer - maybe fine for a commercial buyer, but now fred bloggs also wants some insurance - he needs to now pay tax on it - how are you (the Gov.) going to collect that tax?
I think that you are viewing all of this as a simple equation between UK government and a business (e.g. Amazon in the UK) - however, it is never that simple, there are huge complexities in how a business is / can be structured, and the impact of laws - ultimately, the businesses tend to have cleverer people who get past the increasingly complex machinations of governments - and why not, they have the advantage of being able to chose any location to transact / etc.
That is why my argument is not that the current system works - I agree, it is not ideal - but that the solution is not the communist / left wing approach of penalise those who do well and then we will all be the same - it has been tried, it fails! But the true conservative (with a small c) approach of allowing more freedom and relaxing legislation to make it easier - get businesses to locate here and we will end up with more taxes in the coffers...
I am not sure what relevance that thread has
and no-one is saying that a government can't apply taxes to both parts of a transaction - of course they can (e.g. I sell you a house, I might pay capital gains tax, you might pay stamp duty etc.) - the point being made (and generally ignored!) is that a government can not legislate in a way to force what happens in another territory
the Sauter Shop example is simply the government saying that if you want to sell into this territory you need to do xyz - that is okay because the government governs what happens in this territory...
If you walk into Sauter and buy something, then there is no need for them to register with HMRC as the transaction happens in a territory outside the control of the government - the government can then choose to charge you on importing that item into the UK because that is coming into their territory = their rules... but would then be taxing you as the person bringing it in, not Sauter who are beyond their control.
this whole discussion started with the concept of fees passing across territorial boundaries to avoid taxes in the UK - because those fees are for something intangible, and the transaction takes place outside the UK and is governed under non-UK law - it is beyond the reach of the UK government - it is not in their territory. There is nothing physical coming into the UK to be taxed.